On Tame Sows and Wild Boars

December 30th, 2010 | Meera

When I got to the prep lab this morning, Dave had set out two birds for me to work on: a Savannah sparrow (a species with which I am becoming quite familiar) and a sleek, long-tailed Yellow-billed cuckoo. As the cuckoo thawed, however, it became clear that it had begun to spoil—this happens sometimes when a specimen doesn’t make it to a freezer soon enough after its death—and that it wouldn’t, therefore, make a good study skin. Birds whose tissues are breaking down have skin that falls apart easily, and they inevitably lose a great deal of feathers as you go along. So Dave put the cuckoo back in the freezer to become a skeleton on another day, and drew out as a replacement an exquisitely tiny Golden-winged warbler (Vermivora chrysoptera). As I’ve gotten more comfortable working on small birds, Dave has given me a good many warblers to prepare—but this Golden-winged was the first of its kind that I had ever seen. While he was taking it out of the freezer, he also told me that they aren’t a very common find around here—and so I hoped I would do a good job with the perfect little creature he placed by my tray.

Fortunately, she was kind to me, and turned out beautifully. (I can use the pronoun with confidence because later I placed a pair of magnifying goggles over my head and personally examined the minuscule, very slightly raised, very slightly shinier spot behind her kidneys that Dave, with fantastic authority, indicated as her ovary. To me it looked like a microscopic, colorless oil slick sitting on top of a larger, vaguely less lustrous oil slick. Sexing birds when it isn’t mating season is an exercise in seeing what does not want to be seen.)

But besides being beautiful (Golden-winged warblers have, as their name suggests, bright yellow plumage on their wings—but they also have golden crowns, striking patches of jet black on their faces and throats, and the most modish gray feathers you can imagine cloaking their backs), it turned out that the bird I met today was also a player in a long and fascinating history.

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Here’s how it goes. The first thing you ought to know is that the Golden-winged warbler happens to have a kind of aural twin. One of the main songs you’re apt to hear these birds sing is a two or three-note whistle that is usually described as a high, gentle buzzing, like someone breathing in and out over the surface of a nail file. But hearing that song isn’t always enough to make a positive identification, because another warbler—the much more common Blue-winged—buzzes in a very similar way. And it’s especially apt to do so if it has a little Golden-winged blood in it.

Here’s why it might. The two species of warbler don’t look all that much alike; although they share the same basic colors, the way those colors are distributed on their bodies is quite different. But DNA tests reveal that they’re incredibly similar genetically. In fact, scientists believe that two or three million years ago, Blue-winged and Golden-winged warblers were one and the same. At some point, though, glacier movements across North America caused the population to split into two groups: one isolated somewhere around Missouri, and the other somewhere up in the Appalachians. And from this separation came speciation.

For more than a million years, Golden-winged warblers and Blue-winged warblers were kept apart from each other mostly by dense forests, a habitat in which neither is able to thrive. Eventually, however, humans began clearing those forests and farming them—then abandoning that farmland and allowing a sparser collection of trees and shrubs to grow back. As this change in the landscape occurred, sometime between one and two centuries ago, Golden-winged warblers began moving north and Blue-winged warblers south. And because they still share so much genetic material, when the two meet, they are happy to breed with each other.

If a Golden-winged warbler mates with a Blue-winged, the two produce a reliably identifiable hybrid offspring known as a Brewster’s warbler. (A Brewster’s warbler is then able to mate with either a Golden-winged or a Blue-winged warbler in a process known as a backcross; these pairings, and their successive pairings, produce all manner of other subtly different and unpredictably plumaged birds, as well as—occasionally—another reliably identifiable hybrid known as a Lawrence’s warbler.)

Unfortunately, besides making for an incredibly complicated family tree, this habit of hybridization has spelled a precipitous decline in the Golden-winged warbler population. That’s because Golden-winged warblers are significantly more likely to mate outside their species than Blue-winged warblers, and—much to the chagrin of avian anti-classists, I presume—once they’ve done so, their hybrid offspring aren’t able to find mates as easily as pure-blooded birds. The combination of these two things has meant that, as the species have crossed paths over the past century or so, Blue-winged warblers are making pretty good headway at displacing Golden-winged warblers.

What truly amazes me about all this, beyond the fact that it’s biologically fascinating, is that I know perfectly well the whole story was present in Dave’s head as he handed me that sweet little specimen this morning—even though the only thing he said was “We don’t get too many of these.” Sometimes I wonder what it can possibly be like to know as much as he does about birds; how it must feel to have all this detail stored away inside him as comfortably and naturally as, for instance, you or I might store our feelings about our best friends.

You may have heard, if you live in Chicago and happen to be a newshound, that Dave’s retiring from the Field after 35 years of service. He’ll still be there every day for the next year, but when he does eventually leave it will be a loss beyond words to the division. I can’t tell you how lucky I feel to have spent the past two years working in his lab. And do you know what? If I were that Golden-winged warbler, I’d feel lucky to have passed through it too.

P.S. I owe a debt to Todd McLeish’s excellent Golden Wings & Hairy Toes: Encounters with New England’s Most Imperiled Wildlife.

P.P.S. Anyone curious about the title of this post might want to take a quick look here.

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